A communication apparatus such as a wireless receiver is generally provided with an automatic gain control circuit so that a constant output level can be maintained even when the level of the received signal changes. Particularly when the communication apparatus is a wireless transceiver mounted on a car which is used in circumstances such that the level of a received signal significantly changes, as when passing through tunnels, moving up from a plain to a hill, passing through or between, buildings or approaching the transmitting radio station, the apparatus requires an automatic gain control circuit which can maintain good gain control over a range from a very small received signal level to an unduly large received signal level.
FIG. 10 shows a block diagram of a principal portion of typical wireless receivers which comprise an automatic gain control circuit of the conventional type. This receiver comprises a high-frequency amplifier stage 11 for amplifying a signal received by an antenna 10, a mixer 13 for converting the output of the high-frequency amplifier stage into an intermediate-frequency signal in accordance with a local oscillation signal from a local oscillator 12, an intermediate-frequency amplifier stage 14 for amplifying the intermediate-frequency signal, and a detector stage 15 for demodulating the output of the intermediate-frequency amplifier stage. The detector stage 15 converts the output of the intermediate-frequency amplifier stage 14 into a DC signal to thereby obtain a voltage V.sub.AGC for automatic gain control and feeds it back to the respective amplifier stages 11 and 14. In each of the amplifier stages 11 and 14, the bias of an amplifying element thereof is changed in accordance with the voltage V.sub.AGC to thereby control its gain. A curve V.sub.AGC in FIG. 11 shows one example of the variation of the automatic gain control voltage V.sub.AGC in terms of the variation of the received signal level (RSL) in an automatic gain control circuit having the above structure.
In the case where an automatic gain control circuit of the type whose gain is controlled by changing the bias of an amplifying element thereof is employed as described above, the amount of reduction of the gain of one amplifier stage is limited to about 30 dB due to the influence of parasitic capacitance or the like in its amplifier elements, for example, in a wireless receiver. The gain reduction amount is not sufficient particularly in the case of an unduly large received signal level in a communication apparatus of the aforesaid type which is to be mounted on a car. An automatic gain control circuit of this kind also has a deficiency in that the distortion (d) of the output increases with the reduction of gain as indicated by a curve d in FIG. 11.
Furthermore, an automatic gain control circuit of this type brings about an increase of power consumption in its amplifier stage, if the gain control circuit is of the forward AGC type in which the gain is decreased by changing the bias amount in the forward direction, and is therefore not suitable for use particularly in a portable-type communication apparatus. If the gain control circuit is of the reverse AGC type in which the gain is decreased by changing the bias amount in the reverse direction, a power supply of a different polarity is needed to obtain a sufficient reduction of gain, and this brings about an increase of the manufacturing costs.